Plasmodium, a genus of parasitic protozoans of the sporozoan subclass Coccidia (phylum Apicomplexa) are the causative organisms of malaria. In humans, malaria is caused by P. falciparum, P. malariae, P. ovale, P. vivax and P. knowlesi. (Mueller et al. 2007 and Collins (2012)). Among those infected, P. falciparum is the most common species identified (˜75%) followed by P. vivax (˜20%) (Nadjm and Behrens 2012). Although P. falciparum traditionally accounts for the majority of deaths, (Sarkar et al. 2009) recent evidence suggests that P. vivax malaria is associated with potentially life-threatening conditions (Baird J K (2013)). P. vivax proportionally is more common outside of Africa (Arnott et al. 2012). There have been documented human infections with several species of Plasmodium from higher apes; however, with the exception of P. knowlesi—a zoonotic species that causes malaria in macaques—these are mostly of limited public health importance.
P. falciparum, which is found worldwide in tropical and subtropical areas. It is estimated that every year approximately 1 million people are killed by P. falciparum, especially in Africa where this species predominates. P. falciparum can cause severe malaria because it multiples rapidly in the blood, and can thus cause severe blood loss (anemia). In addition, the parasite infected red blood cells can clog small blood vessels. When this occurs in the brain, cerebral malaria results, a complication that can be fatal.
P. vivax, which is found mostly in Asia, Latin America, and in some parts of Africa. Because of the population densities especially in Asia it is probably the most prevalent human malaria parasite. P. vivax (as well as P. ovale) has dormant liver stages (“hypnozoites”) that can activate and invade the blood (“relapse”) several months or years after the infecting mosquito bite.
P. ovale is found mostly in Africa (especially West Africa) and the islands of the western Pacific. It is biologically and morphologically very similar to P. vivax. However, differently from P. vivax, it can infect individuals who are negative for the Duffy blood group, which is the case for many residents of sub-Saharan Africa. This explains the greater prevalence of P. ovale (rather than P. vivax) in most of Africa.
P. malariae, found worldwide, is the only human malaria parasite species that has a quartan cycle (three-day cycle). (The three other species have a tertian, two-day cycle.) If untreated, P. malariae causes a long-lasting, chronic infection that in some cases can last a lifetime. In some chronically infected patients P. malariae can cause serious complications such as the nephrotic syndrome.
P. knowlesi is found throughout Southeast Asia as a natural pathogen of long-tailed and pig-tailed macaques. It has recently been shown to be a significant cause of zoonotic malaria in that region, particularly in Malaysia. P. knowlesi has a 24-hour replication cycle and so can rapidly progress from an uncomplicated to a severe infection; fatal cases have been reported.
In addition to these Plasmodium-mediated diseases, additional parasitic diseases are caused by protozoa as Leishmania, Trypanosoma, Entamoeba, Giardia, Naegleria, and Trichomonas. Taken together, these protozoan parasitic diseases are responsible for more than three millions deaths annually throughout the world. The WHO has declared six major diseases namely leishmaniasis, malaria, amoebiasis, filariasis, Chagas disease and schistosomiasis in its Special Programme for Research and Training in Tropical Diseases. Selectivity of an antiparasitic compound should target as its mode of specific inhibition an aspect that leaves host processes unaffected.
Currently, Artemisinin combination therapy (ACT) is the treatment of choice for P. falciparum malaria. Resistance to ACT therapy has recently been documented in Southeast Asia. It is likely only a matter of time before ACT resistance spreads and diminishes its efficacy as a first line therapy for malaria. Thus, it is imperative to identify and develop novel antimalarial therapies to treat the hundreds of millions of people infected by malaria each year. There is also a need for drugs that can be taken prophylactically when travelling to malaria endemic regions. Of note, some of the medicines currently available for prophylaxis of travelers to malaria endemic regions have significant side effects or are not suitable for use by children or pregnant woman. Thus, improved drugs are needed to address these unmet needs.
Some eukaryotic parasites, such as Plasmodium species that cause malaria, Leshmania species that cause leshmaniasis, Trypanasoma species that cause African sleeping sickness and Chagas disease and Toxoplasma gondii that causes toxoplasmosis, are purine auxotrophs, unable to perform de novo purine biosynthesis (Carter, N. S. et al. (2008); Landfear, S. M. et al. (2004); Cass, C. E. et al. (1998); Cass, C. E. et al. (1999); and Cassera, M. B. et al. (2011)). Because nucleobases and nucleosides are impermeable through phospholipid cell membranes, cells use Equilibrative Nucleoside Transporters (ENTs) and Concentrative Nucleoside Transporters (CNTs) to import and export purines and pyrimidines (Baldwin, S. A. et al. (2004), and Pastor-Anglada, M. et al. (2008)). The parasites rely on purine import via ENTs, and possibly CNTs, to supply purines needed for DNA synthesis and other cellular processes. The imported purines are processed through the purine salvage pathway to generate the specific purines required by the cell.
Blocking purine import will have inhibitory or cytotoxic effects on these parasites. For example, knockout of the Plasmodium falciparum Equilibrative Nucleoside Transporter Type 1 (PfENT1) results in parasites that are not viable during in vitro culture in growth media containing physiological purine concentrations found in normal human blood (El Bissati, K. et al. (2006)).
The present invention discloses ENT inhibitors and their use as anti-parasitic compounds.